FIG. 1 is a cross sectional view to show a constitution of a current passing device of a DC motor. FIG. 2 illustrates a flow of current in the current passing device in FIG. 1. In the drawing is illustrated a longitudinal cross sectional view to show an EGR (Exhaust Gas Recirculation) valve means provided as a driving source. In FIG. 1, a reference numeral 1 denotes a valve body in which a passage communicating with an exhaust gas return passage c disposed in a recirculation system of an exhaust gas. When a control valve 11 is moved up as shown in the drawing, it is put into contact with a seat 12 to thereby close the exhaust gas return passage c and when the control valve 11 is moved down, it is separated from the seat 12 to thereby open the exhaust gas return passage c.
A reference numeral 21 denotes a DC motor to become a driving means (torque generating source) of the control valve for opening/closing an exhaust gas passage, 22 denotes a motor case mounted on the valve body 1, 23 denotes a rotor which is rotatably supported in the motor case 22 via bearings 24, 25, and the rotor 23 has a screw hole 23a made through a shaft central portion. A reference numeral 26 denotes a magnet mounted on the outer peripheral portion of the rotor 3, 27 denotes a stator core surrounding the outer periphery of the magnet 26, and a predetermined gap is formed between this stator core 27 and the above mentioned magnet 26.
A reference numeral 28 denotes a coil arranged in the stator core 27 and constituting a stator, 29 denotes a motor shaft to be a valve driving shaft, and this motor shaft 29 is made of a screw shaft and screwed into the screw hole 23a of the above mentioned rotor 23 and moved in an axial direction by a rotation of the above mentioned rotor 23.
A reference numeral 30 denotes a valve shaft abutting member which is integrally formed with the motor shaft 29. By making the top end of a valve shaft 13 having the above mentioned control valve 11 abut against the tip end (bottom end in FIG. 1) of the valve shaft abutting member 30, the valve shaft 13 is made to follow a movement in the axial direction of the above mentioned motor shaft 29 to thereby open/close the above mentioned control valve 11.
A reference numeral 31 denotes a cover mounted on an end surface on the valve shaft abutting member 30 side in the motor case 22, 32 denotes a spring retaining member mounted on the tip end side of the valve shaft abutting member 30, 33 denotes a spring which is hung between the spring retaining member 32 and the above mentioned cover 31 for urging the valve, and this spring 33 urges the control valve 11 at the tip end of the valve shaft via the valve shaft abutting member 30 in a direction that opens the control valve 11. A reference numeral 34 denotes a commutator rotating integrally with the rotor 23 and has a shaft hole 34a at a central portion. A reference numeral 14 denotes a spring receiver mounted on the top end of the valve shaft 13 and a return spring 18 is mounted between the spring receiver 14 and the valve body 1.
A reference numeral 35 denotes a power source side case mounted on an opening end portion on the commutator 34 side of the motor case 22. This power source side case 35 is an integrally molded part made of synthetic resin and constitutes a current passing device body of the DC motor 21 and is an integral combination of a built-in sensor portion 36 in which a position sensor (not shown) for detecting an opening of the above mentioned control valve is built and an input/output connector portion 37 in which a connector terminal is built.
A reference numeral 38 denotes a brush holding cylindrical portion integrally molded with a wall opposed to the commutator 34 in the power source side case 35, and 39 denotes a brush slidably inserted into the brush holding cylindrical portion 38, for passing a current. This brush 39 is made of carbon powder and copper powder and is connected to the connector terminal of the above mentioned input/output connector portion 37 via a lead wire (not shown).
Next, an operation will be described. Passing a current through the coil 28 via the brush 39 and the commutator 34 generates a rotational torque in the rotor 23 by an interaction between a magnetic field generated in the coil 28 and a magnetic field of the magnet 26. When the rotor 23 is rotated by the rotational torque, the motor shaft 29 screwed into the screw hole 23a of the rotor 23 is fed by the screw and moved straight in an axial direction.
Here, in a case where the motor shaft 29 is moved downward in FIG. 1, the valve shaft abutting member 30 is pushed by the motor shaft 29 in a direction of urging force of the spring 33 to thereby open the control valve 11 via the valve shaft 13 against which the valve shaft abutting member 30 abuts. Then, when the control valve 11 moves near to a target position and a difference between a present position and a target position becomes nearly equal to zero, an electric power required to hold the valve 11 at its position against the restoring force of the return spring 18 is supplied to the DC motor 21.
On the other hand, when the rotor 23 of the DC motor 21 is rotated in a direction opposite to the above mentioned direction, the motor shaft 29 is moved upward in FIG. 1 and the valve shaft 13 abutting against the valve shaft abutting member 30 which is made integrally with the motor shaft 29 follows to and moves with the motor shaft 29 against the restoring force of the return spring 18 to thereby close the valve.
FIG. 2 is a diagram of one example of a control circuit for supplying the DC motor 21 with a control signal. A reference numeral 41 denotes a position sensor (position detecting means) and the position sensor 41 has a movable contact portion 43 moving on a resistor 42 to which a constant voltage is applied by a power supply terminal and when the movable contact portion 43 moves with the rotation of the rotor 23, a voltage responsive to its movement position is outputted as a detection signal.
A reference numeral 44 denotes a controller block and the controller block 44 has an A/D input port 45 for inputting the detection signal from the valve position sensor 41, a filter 46 for reading an output of the A/D input port 45, a PI control computing section 48 for performing a PI control computation based on an output of the filter 46 and an output of a target value determining section 47, an excitation switching and duty computing section 49 for performing an excitation duty computation based on an output of the PI control computing section 48, a digital output port 50 for outputting an ON/OFF signal and a PWM output port 51 for outputting a PWM signal, on the basis of an output of the excitation switching and duty computing section 49, and an external operation input section 61.
A reference numeral 52 denotes a switching circuit for controlling the passing of a current through the DC motor 21. A (+) side end of the DC motor 21 is connected to a connection point of semiconductor switching devices (hereinafter referred to as switch device) 53, 54 connected in series and a (−) side end thereof is connected to a connection point of semiconductor switching devices (hereinafter referred to as switch device) 55, 56 connected in series.
To each of the above mentioned switch devices 53, 55 is connected each of transistors 57, 58 which are turned ON/OFF by an output of the digital output port 50, and to each of the above mentioned switch devices 54, 56 is connected each of transistors 59, 60 turned ON/OFF by an output of the PWM output port 51.
Next, an operation will be described.
When a target value is determined by the target value determining section 47, the PI control computing section 48 performs the PI control computation based on a present value which is detected by the position sensor 41 and inputted via the A/D input port 45 and the filter 46, and the above mentioned target value, and the excitation switching and based on the computation result duty computing section 49 performs the excitation duty computation.
The digital output port 50 outputs an ON signal to terminal a and terminal b based on the output of the excitation switching and duty computing section 49 to bring the switch device 53 into conduction via the transistor 57, for example, to thereby connect the (+) terminal of the DC motor 21 to a power applying terminal V. On the other hand, the PWM output port 51 outputs a PWM signal to the terminal a and terminal b based on the output of the excitation switching and duty computing section 49 to control a conduction of the switch device 56 via the transistor 60, for example, to thereby pass a current in a direction shown by a solid line arrow through the DC motor 21.
In this manner, the DC motor 21 is operated, the motor shaft 29 is moved down by the rotation of its rotor 23, and by this moving down motion, the valve shaft 13 is moved in the same direction against the return spring 18 to move the control valve 11 to a target position to thereby open the control valve 11. Then, when the control valve 11 moves near to the target position and a difference between the present value and the target value becomes nearly equal to zero, an electric power only required enough to hold the control valve 11 at its position against the restoring force of the return spring 18, is supplied to the DC motor 21.
Next, in the above mentioned state in which the valve is opened, when the outputs of the terminal a and terminal b of the digital output port 50 and the PWM output port 51 are reversed to bring the switch devices 54, 55 into conduction via the switch devices 58, 59, a current is passed through the DC motor 21 in a direction shown by a dotted line to rotate the rotor 23 of the DC motor 21 in a reverse direction. In this manner, the motor shaft 29 is moved upward in FIG. 1 and the valve shaft 13 follows this movement and is moved by the urging force of the return spring 18 to thereby move the control valve in a direction that closes the valve. Then, when the control valve 11 abuts against the seat 12 and is closed, the valve shaft 13 stops moving but the motor shaft 29 continues to move until the DC motor 21 stops and abuts against a stopper S and stops there.
Since a motor shaft in a conventional DC motor is constituted in such a way that it is made to abut against the stopper to be stopped as described above, a response of the DC motor varies depending on a kind of braking and a judgment for braking start. In particular, in a case of the DC motor having high response, there is presented a problem that the motor shaft collides with the stopper at a motor stop point to cause breakage because of an overshoot of the rotor.
The present invention has been made to solve the above mentioned problem, and an object of the present invention is to provide a braking device of the DC motor capable of reducing a collision of the motor shaft with a stopper at a motor stop point and stopping the motor shaft quickly.